In most modern transceivers, for both technical and cost reasons a single frequency reference is used for the reference clock and the carrier oscillator in the transmitter, and a single frequency reference is used for the reference clock and the carrier reference in the receiver. The carrier oscillator and the carrier reference are respectively generated by multiplying the respective frequency reference by a predetermined value.
In the transmitter the reference clock is used as a sample clock in the modulator and in the digital-to-analogue converter, and in the receiver the reference clock is used in the analogue-to-digital converter and in digital signal processing circuitry (including the demodulator).
In such transceiver designs, in which part of the functionality takes place in discrete time, it is required to synchronise the sample timing of the receiver with the sample timing of the transmitter, i.e. synchronise the reference clock of the receiver with that of the transmitter. There are known techniques to derive sample tracking out of the received signal. However, such known techniques require complex circuitry in the receiver to ensure timing tracking of the carrier.
A method is usually provided in the receiver to synchronise the receiver with the carrier on which the received signal is modulated. This synchronisation involves determining, and removing, the error between the carrier frequency of the transmitter and the reference carrier frequency in the receiver. Especially in coherent detection receivers, the receiver must be very rigidly synchronised, or locked, to the received carrier.